Dynamics of CrO₃–Fe₂O₃ Catalysts during the High-Temperature Water-Gas Shift Reaction: Molecular Structures and Reactivity

Abstract

A series of supported CrO3/Fe2O3 catalysts were investigated for the high temperature water-gas shift (WGS) and reverse-WGS reactions and extensively characterized using in situ and operando IR, Raman and XAS spectroscopy during the high temperature WGS/RWGS reactions. The in situ spectroscopy examinations reveal that the initial oxidized catalysts contain surface dioxo (O=)2Cr +6 O2 species and a bulk Fe2O3 phase containing some Cr 3+ substituted into the iron oxide bulk lattice. Operando spectroscopy studies during the high temperature WGS/RWGS reactions show that the catalyst transforms during reaction. The crystalline Fe2O3 bulk phase becomes Fe3O4 and surface dioxo (O=)2Cr +6 O2 species are reduced and mostly dissolve into the iron oxide bulk lattice. Consequently, the chromium-iron oxide catalyst surface is dominated by FeOx sites, but some minor reduced surface chromia sites are also retained. The Fe3-xCrxO4 solid solution stabilizes the iron oxide phase from reducing to metallic Fe o and imparts an enhanced surface area to the catalyst. Isotopic exchange studies with C 16 O2/H2 → C 18 O2/H2 isotopic switch directly show that the RWGS reaction proceeds via the redox mechanism and only O* from the surface region of the chromium-iron oxide catalysts are involved in the RWGS reaction. The number of redox O* sites were quantitatively determined with the isotope exchange measurements under appropriate WGS conditions and demonstrated that previous methods have undercounted the number of sites by nearly an order of magnitude. The TOF values suggest that only the redox O* sites affiliated with iron oxide are catalytic active sites for WGS/RWGS, though a carbonate oxygen exchange mechanism was demonstrated to exist, and that chromia is only a textural promoter that increases the number of catalytic active sites without any chemical promotion effect.

Operando Spectroscopic Study of Dynamic Structure of Iron Oxide Catalysts during CO2 Hydrogenation

“…[6][7][8] By using NAP-XPS and operando techniques, the mechanisms of Fe-based catalysts during hight emperature RWGS and WGS process have been revealed. [9,10] However, for the CTH process, both the structure of active phases and the structure-performance relationship of Fe-based catalysts are still ambiguous. [10,11] In particular, the origin of active sites of Fe-based catalysts and specific effects of different iron species on the product distributions are yet to be fully understood.…”

Cr-Free, Cu Promoted Fe Oxide-Based Catalysts for High-Temperature Water-Gas Shift (HT-WGS) Reaction

“…Activated catalysts: After activation of the catalysts under WGS reaction mixture, the bulk Fe 2 O 3 phase reduces to form Fe 3 O 4 phase [6][7][8][9]. Interestingly, the dissolution limit of various promoters changes significantly in the Fe 3 O 4 lattice compared to the Fe 2 O 3 lattice [29].…”

Atomic Scale Materials for Emerging Robust Catalysis

“…ASMs are not only widely used in the fundamental catalytic reaction systems, but are also gradually applied to the industrial application systems, such as WGSR. WGSR is an essential process referring to the transformation from H 2 O and CO to H 2 and CO 2 . The operated condition at a low temperature is beneficial to drive the thermodynamic process of WGSR due to the intrinsic exothermic reaction.…”